1. Field of the Invention
The present invention relates generally to the field of thin-film sol-gel coatings and in particular to coating on flat substrates such as glass or solar panels.
2. Description of Related Art
Thin-film sol-gel coating refers to a technique of coating substrates using a wet chemical formulation called a ‘sol’ that undergoes a ‘gelation’ process wherein it polymerizes to form a solid thin-film on a substrate. It is a very versatile process that has many industrial uses such as formation of dielectric layers on semiconductor wafers and water repellent layers on ceramics. There are several well documented techniques for applying wet sol to substrates, some of which are in widespread industry use and others that have generally been limited to the laboratory. Industrial scale sol-gel coating is most commonly performed by a dip, spray, spin, meniscus, or roller process.
In the dip coating process a substrate to be coated is dipped into a tank containing the sol. It is then withdrawn at a process dependent speed. As the substrate is slowly drawn from the sol, the gelation process occurs just above the surface and a thin-film layer forms. The major limitations with the dip coating process are first the fact that it is inherently two sided, that all sides and edges of the substrate are coated. This can be advantageous in some cases but is disadvantageous if the coating on some portion of the substrate interferes with a downstream process step. Second, it requires a tank slightly larger than the substrate. For large substrates this means the tank must hold a significant volume of sol. It is not unusual for sol to be mainly composed of an organic solvent, so large quantities pose a vapor and flammability hazard. Third, it can be challenging to control the composition and quality of the sol within the tank. Each new substrate dipped in the tank may carry contamination that is transferred to the sol; the sol might become depleted in some element as more substrates are processed, causing a variation in the thin-film produced, and the sol may change through evaporation of solvent at the surface where substrates are introduced.
Spray coating exists in many forms, but generally may be considered to be the deposition of material through a nozzle under pressure or the atomization of material which is then entrained by a jet of air. In all cases the material is moved across an airspace gap between a nozzle and a surface to be coated. The purpose of the spray system is to deposit an approximately uniform layer of material over a wide area of the substrate. In the context of sol-gel coatings on substrates spray coating has the advantage of only applying fresh material to the substrate. However, spray coating has some major limitations. First, generally a large amount of material has to be sprayed to get uniform coverage over a large area. Second, solvent within the sol may evaporate during the travel time from the nozzle to the substrate—changing the concentration of the sol deposited. Typically this limits the kinds of solvents that can be used to those with lower volatility. Third, spraying requires that either the nozzle or the substrate be moved in order to coat an area—for example the substrate may be moved past a line of stationary nozzles—which increases the complexity of the overall system. It has also been generally reported that it is difficult to achieve completely uniform coatings with this method.
Spin coating is commonly used in the semiconductor wafer processing industry and in the LCD display panel industry to apply even layers of material to the surface of flat substrates such as silicon wafers or large pieces of glass. It has the same advantage as spray coating in that only fresh material is deposited. It also has excellent uniformity control. However, equipment to perform the spin coating is necessarily complex and costly to maintain because of the fine mechanical control needed to achieve that uniformity. This is particularly true as the size of the substrate increases.
Meniscus coating was used during the early 1990's in the semiconductor industry before giving way to spin coating. It remains in use by some equipment vendors in the LCD display industry. Meniscus coating works by passing a substrate to be coated over a narrow slot at a very close distance such that material forced up through the slot forms a meniscus with the substrate. As the substrate moves across the slot this meniscus deposits a layer of material on the substrate. The technique requires fine control over the distance between the slot and the substrate across the full length of the slot. Generally the substrate must be extremely flat to avoid deviation in this distance. Additionally this technique works best with more viscous materials that can form a larger meniscus which limits it usability with sol-gel formulations that use non-viscous solvents.
Roll coating is in common usage for sol-gel coatings on flat substrates. In this process material is deposited from a reservoir onto a feed roller. A doctor blade may be used to control the thickness of material on the roller. That material is then transferred to a coating roller that is in contact with the substrate. In general roll coating works best with continuous substrates, such as for example a roll of steel. In the case of discontinuous substrates such as pieces of glass for example it is possible to get variation at the starting edge of the piece as coating roller reaches equilibrium. This is to say the first one or two revolutions of the coating roller on the substrate may deposit a slightly different thickness compared to subsequent revolutions. Additionally, the surface of the roller is often a compliant material that serves to compensate for any surface or flatness imperfections on the substrate and to provide a surface to which the coating material will adhere in a reasonably uniform manner. Unfortunately chemical compatibility between this surface material and the sol-gel formulation can limit versatility of this coating method.
Given the various limitations of the current coating methods for sol-gel coating of flat substrates there is clearly a need for an industrial scale method that might have many of the following attributes. The ability to selectively coat just one face of a substrate, that only deposits fresh coating material, that does not require compositional or contamination control of large tanks of material, that is economical with material deposition, that is versatile with respect to the sol-gel formulation such that solvents of different volatilities can be used and chemical compatibilities with critical equipment are relaxed, that is of low complexity and cost, that can handle large imperfections in substrate surface flatness, and that can achieve superior coating uniformity.